1. Field of the Invention
The present invention relates to a tomography device and process with a high acquisition rate, which can be used especially in medicine.
It relates generally to reconstructed image tomographs using digital processing of acquired data. The main purpose of the invention is to reduce the waiting time, especially for patients, entailed by the acquisition of multiple-section mode images or 3D images. The invention is also designed as a complementary installation for radiotherapy simulation machines. For machines of this type, the main constraint lies in the need to enable the practitioner to reach any organ of a patient at any moment, even during the tomography acquisition stage.
2. Brief Description of the Prior Art
There are known methods for reconstructed image tomographs wherein a set comprising a multidetector and an X-ray tube, fixed with respect to each other in the same plane, is used to acquire data relating to the image of a section, along this plane, of an object interposed between this X-ray tube and this multidetector. As a matter of fact, for each orientation of a particular direction of a fan beam radiation from the X-ray tube with respect to the object, it is possible to acquire a set of data representing a view of a slice of this object along this orientation. By repeating the experiment for different orientations, it is possible to acquire a set of views from which, under certain circumstances, a processing algorithm, commonly called a back projection algorithm, can be used to reconstruct the image of the section of the object along this plane.
The total time for acquisition of the data that enables the reconstruction of this image depends on the rotation speed of the multiple detector/X-ray tube set, the power of the X-ray tube and the sensitivity of the detector. Intuitively, it can be understood that the greater the power of the X-ray tube, the more it will emit X-rays of a given hardness (and not harder X-rays), the easier it will be to detect the residue of these rays, with the multidetector, after they have passed through the regions of the object, including the most opaque regions. It will then be possible to make the tube/detector set rotate more quickly to acquire the image more quickly. Following the same line of thought, the more sensitive the detector, the greater its quantum rate, and the more it is possible to increase its acquisition speed. This being the case, for a given technology of the X-ray tube/multiple detector set, the rotation speed of the set is a determining factor in the quality of the reconstructed image. For the lower the speed, the more possible it is to leave the multiple detectors under prolonged influence of the X-ray tube at each view, and the more precise and typical will be the data delivered by each of the cells of this detector. By contrast, the faster the operation, the more the image will deteriorate in terms of precision or resolution.
Currently available reconstructed image tomography machines are mainly so-called third generation machines. In these machines, the X-ray tube/multiple detector set, although it rotates, is connected by cables to the supply and receiving circuits of the machine. An acquisition then requires the acceleration of the X-ray tube/multiple detector set when starting up, the stabilizing, in speed, of this set, the acquisition proper of data at a stabilized speed, and the slowing down and then the stoppage of set (before the cable connections are pulled out). All this movement in rotation means that the set must perform two rotations on itself. For another acquisition, for the image of a section adjacent to a previously acquired section, this set of operations is started again, making the machine rotate in the other direction, and so on. In this operating mode, the result is that the acquisition time, in practice, is not related solely to the above-mentioned choices as regards image quality but is related above all to this type of operation. In practice, the duration itself of acquisition varies from between half a second for images requiring low resolution up to ten seconds for images with fine resolution. In both cases, the acquisition of data relating to a section, taking acceleration and slowing down into account, is about 10 seconds.
The most commonly known multiple detectors are gas multiple detectors comprising a set of cells filled with gas (generally Xenon). Each cell of the multidetector has two metallic plates which are electrically biased at reverse voltages with respect to each other. The X-radiation, which goes through the object, causes this gas to be ionized and allows a leakage current to flow between the two plates biased at opposite high voltages. By measuring these leakage currents, it is possible to detect the data needed to reconstruct the image. A multidetector therefore has a great number of cells of the same type, aligned behind one another in a row, the main direction of which is contained in the section plane of the X-ray tube/multiple detector set. Cells of this type have a width of about 1.5 cm., measured perpendicularly to this sectional plane. So as to acquire the finest possible images, a collimator is used, the purpose of which is to reduce the omnidirectional radiation of the tube to radiation in a flat, fan-shaped beam which illuminates the entire detector and has a width corresponding to the resolution pitch, in terms of thickness, of the section to be reconstructed. In practice, this thickness may vary, from one machine to another, from 1 mm to 10 mm. When this thickness is equal to 10 mm, the delivery rate of the X-rays to the multidetector can reach a sensitive level more quickly, even for the most opaque regions of the object, so that such images, which are the poorest ones in terms of resolution, are, however, the most swiftly acquired images.
When it is sought to make a 3D depiction of the objects studied, images of several adjacent sections of this object are made. This is obtained by shifting the object, after making each section, along an axis perpendicular to the plane of the section with a length equal to the thickness of the beam. In one example, if 3D knowledge of an object is sought, along a length of 25 cm of this object with a resolution of 1 mm in the direction of this length, 250 adjacent sections have to be made. For medium resolution, the rotation (acceleration, stabilization, acquisition and slowing down) period, including the 1-mm lateral translations each image, may be about 4 seconds. The total acquisition time is then about 1000 seconds.
The tube/detector set thus described, taking into account the number of rotations to be made at each acquisition, should be contained in a fixed, circular cowl that prevents operators working on this machine from being injured. The various sections are acquired by shifting this cowl with respect to an examination bed on which the object (namely the patient, in medicine) is placed. In practice, the cowl is fixed to the ground and the bed moves by longitudinal translation in the machine. A circular cowl of this type does not give the practitioner access at all times to all the organs of a patient. For example, if the patient undergoes an examination in the region of the heart, his head is on one side of the machine while the lower part of the trunk and the lower limbs are on the other side. This disadvantage is particularly troublesome if the tomograph is used in a machine known as an radiotherapy simulator.
The purpose of a machine of this kind is to give data about the internal and external anatomy of a patient in order to determine, in advance, the characteristics of the beams which will be used when this patient is subsequently placed in a radiotherapy machine, so that the X-rays are given in the required dose to the regions that have to be treated and in the smallest possible dose to the other parts of this patient.
For examination using the radiotherapy simulator, the patient should be placed in the exact position that he would take in the radiotherapy machine. These positions are codified to give the maximum efficiency to the treatment. They are often incompatible with the geometry of conventional tomography machines, all the more so as the operator has to help the patient take these positions. Furthermore, during the examination using the simulator, the operator has to draw marks on the patient's skin to enable the positioning of the radiotherapy beams. This means that the neighbourhood of the patient should be easily accessible during the examination in the simulator.
Nor does removing the cowl, which is itself bulky, solve the problem for, when this cowl is not there, the operators and the patient run the risk of being caught up in the machine. To prevent this risky situation, speed limits have been laid down by the authorities. The purpose of these limits is restrict movements, without cowls, in such a way that source/multiple detector cannot make more than one rotation on itself in less than 60 seconds. Thus, the acquisition of tomographies in order to reconstruct a 3D depiction of an object is inconceivable.
In the invention, these drawbacks are coped with through the realization that, when the number of sections required is great (and especially when the longitudinal resolution required is high), the use of reconstructed image tomographs, as stipulated until now, has not been the most efficient one. Rather than acquiring each image through a group of views obtained by a rotation of the set around the object, and rather than causing each acquisition to be followed by an elementary translation to acquire a following image, the idea, on the contrary, is to cause a longitudinal scan of the object by the tube/detector set, and to cause each translation to be followed by a one-step rotation. At each rotational step, the object is explored by a longitudinal scan. Preferably, in an improvement, the scanning is done downwards (in the head-to-foot direction) and then upwards (in the foot-to-head direction) at the following rotation step and so on.
The comparative magnitudes are as follows: in the prior art, 250 images each comprising 1000 views and each lasting four seconds, including the translation between images, resulted in a total acquisition time of 1000 seconds. With the method of the invention, the procedure comprises a corresponding number of 000 translations, each followed by a rotational movement of one step by the X-ray tube/multiple detector set. It should be noted that these step-by-step rotational movements have very small amplitude; they can therefore be very fast. In fact, their amplitude is about 18 angular minutes (to 1 angular degree depending on the quality of the examination). Their fast execution cannot entail any risk to operators close to the machine. Similarly translations along about 30 cm (25 cm of exploration and twice 2.5 cm of acceleration and slowing down on either side) can also be fast. Thus, it is possible to achieve a total acquisition time of 1000 seconds. However, in this case, the method according to the invention has the advantage wherein, since the accelerations and slowing down of the set are relatively not as long, the relative time taken for the use of the multidetector for detection purposes may be lengthier. Finally, the sensitivity of the machine is thereby increased. For example if the longitudinal scanning time is about 0.75 seconds for 25 cm, the radiation time of the multidetector with respect to a one-millimeter thick slice is about 3 milliseconds (0.750/250). By comparison, for third generation machines, either this using time is shorter in order to keep the total acquisition time to the same length, or this intrinsic using time is the same but, in this case, the total acquisition time is longer.
Moreover, in an improvement, these results, which are again partly comparable, can be considerably increased in the invention if the multidetector is of the multiple row type. For, it is possible to choose a multidetector of the type described in the European patent No. 0 051 350 filed on 28th July 1981.
This multidetector has a two-dimensional array of detecting cells. In this case, the processing is of the discrete type. During the translation, the electrical charges, representing data from the cells of one of the rows of the multiple row multidetector, are transferred to adjacent cells of an adjacent row in the reverse direction to the translation movement. The data content of the cells of the last row takes into account the integration effect resulting from these transfers. It is used as being typical of the view.
To simplify the description, it may be indicated that, beneath a given slice of the object, there is a flow, in order, owing to the movement, of the rows of cells numbered 1 to n (n being equal, for example, to 5). Each time that another row is placed under the slice, the data content of the cells of the previous row is transferred into the cells of this row, and so on until the row number n of the cells is under the slice. At the end of the irradiation of this row beneath this slice, the signal is sampled. This method has the advantage, finally, of subjecting the examined slice to radiation which is n times longer. The immediate result of this is that, for equal resolution quality, the image can then be acquired n times faster; or else at equal acquisition speed, its resolution may be n times better.
Moreover, the device according to the invention wholly complies with the legal requirements referred to above. For, at each rotation by a small angle, of a fraction of a degree, there is no reason to take too many precautions. The motion can therefore be very swift because it has, moreover, a very small span. Besides, the translational movements, of about 30 cm, can be done inside a cowl. In fact, on 30 cm, an operator could be injured if he were caught up by the machine. But since, in the invention, complete rotations are no longer done as in third generation machines, it is no longer necessary to create a circular type of structure for this cowl. A structure with a general shape of an arc of a circle would be enough. Through the inside of the arc of the circle it then becomes possible to reach all the organs of the patient.